The American Cancer Society estimates that 54,000 new cases of in situ breast cancer representing 20 percent of new breast cancers will be diagnosed in 2002. With incidence of ductal carcinoma in situ (DCIS) increasing rapidly, primarily due to greater mammography use, epidemiologic research to identify risk factors that can contribute to prevention is essential. The proposed case-control study will evaluate risk associated with teenage and adult active and passive tobacco smoking, timing and patterns of alcohol consumption, pharmaceutical hormone use, physical activity, breastfeeding duration, nonsteroidal anti-inflammatory drug use, and exposure to home and wide-area pesticide use and wastewater-infiltrated tap water. Mammography use will be assessed in order to take into account any exposure-related screening behaviors. Medical records will be reviewed for information on nuclear grade and presence of comedo necrosis. Study participants will be 224 cases diagnosed among Cape Cod residents in 1992-1998, the earliest and most recent years available from the MA Cancer Registry, and a comparison group of Cape residents similar in age. Exposures will be assessed using interviews and a geographic information system (GIS) containing environmental data, modeling tools to estimate historical exposure, and study participants' addresses dating back to 1948. The study builds on the Cape Cod Breast Cancer and Environment Study of invasive breast cancer and additional grants that currently support the proposed study of DCIS.